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Add USB debugging section to FAQ

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Joël Schulz-Ansres 2024-04-22 00:38:09 +02:00
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docs/modules/ROOT/pages/faq.adoc
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@ -232,6 +232,26 @@ Look for the `MEMORY` section and try to determine the FLASH and RAM sizes and s
If you find a case where the memory.x is wrong, please report it on [this Github issue](https://github.com/embassy-rs/stm32-data/issues/301) so other users are not caught by surprise.
== The USB examples are not working on my board, is there anything else I need to configure?
If you are trying out the USB examples and your device doesn not connect, the most common issues are listed below.
=== Incorrect RCC config
Check your board and crystal/oscillator, in particular make sure that `HSE` is set to the correct value, e.g. `8_000_000` Hertz if your board does indeed run on a 8 MHz oscillator.
=== VBUS detection on STM32 platform
The USB specification requires that all USB devices monitor the bus for detection of plugging/unplugging actions. The devices must pull-up the D+ or D- lane as soon as the host supplies VBUS.
See the docs, for example at link:https://docs.embassy.dev/embassy-stm32/git/stm32f401vc/usb/struct.Config.html[`usb/struct.Config.html`] for information on how to enable/disable `vbus_detection`.
When the device is powered only from the USB bus that simultaneously serves as the data connection, this is optional. (If there's no power in VBUS the device would be off anyway, so it's safe to always assume there's power in VBUS, i.e. the USB cable is always plugged in). If your device doesn't have the required connections in place to allow VBUS sensing (see below), then this option needs to be set to `false` to work.
When the device is powered from another power source and therefore can stay powered through USB cable plug/unplug events, then this must be implemented and `vbus_detection` MUST be set to `true`.
If your board is powered from the USB and you are unsure whether it supports `vbus_detection`, consult the schematics of your board to see if VBUS is connected to PA9 for USB Full Speed or PB13 for USB High Speed, vice versa, possibly with a voltage divider. When designing your own hardware, see ST application note AN4879 (in particular section 2.6) and the reference manual of your specific chip for more details.
== Known issues (details and/or mitigations)
These are issues that are commonly reported. Help wanted fixing them, or improving the UX when possible!